Purification of anhydrous hydrogen fluoride



fiatented Oct. 17, 1950 UNITED STATES PATENT OFFICE PURIFICATION OFANHYDROUS HYDROGEN FLUORIDE Wilbur J. Shenk, Jr., and George R. Pellon,Cleveland, Ohio, assignors to The Harshaw Chemical Company, Cleveland,Ohio, a corporation of Ohio No Drawing. Application July 14, 1948,

Serial No. 38,757

The anhydrous hydrogen fluoride of commerce so produced containssubstantial quantities of 7 water, S02 and H23, and sometimesalso'contains small quantities of organic sulfur compounds, probablyderived mainly from the flotation agent used in preparing the fluorsparused for acid production. Water may be present in proportions below0.5%, usually around 0.1% by Weight. SO2

and H25 together may be present to the extent of two or three-tenths of1% by weight and organic sulfur compounds in lesser proportions.

It is known that pure anhydrous HF is very inert with respect to steelsuch as steel shipping drums and steel equipment as well as to othermetals such as nickel and copper. It is known, also, that an increase inwater content and S02 content reduce the inertness and render theproduct more corrosive. It is obvious, then, that if the anhydroushydrogen fluoride of commerce could be freed of its water and sulfurcontent, it would be improved withrespect to its transportation and use.It has been proposed heretofore to purify the anhydrous hydrogenfluoride of commerce by redistillation for the reduction of the contentof moisture and sulfur; but it has been found that even afterredistillation, there still remains a small proportion of the S02 orpounds, SOzFz and SFs, the water content beingat the same time convertedto HF and oxygen. In accordance with the present invention, S02 and H25are fluorinated to produce the very stable sulfur-fluorine compoundsSO2F2 .and SFs which are exceedingly inert as is Well known.

4 Claims. (01. 23-453) l i Y z Fluorination at the same time convertsthe moisture content to hydrogen fluoride and oxygen,

and the sulfur in the organic sulfur compounds,

which may be present in trace quantities, is con verted to SOzFz or SP6.In View of the fact that the fluorination of small quantities ofimpurities is carried. out inlarge quantities of anhydrous HF, it isclear that fluorine gas cannot very well be employed as the fluorinatingagent, since traces would remain and contaminate the prod uct.

It has been found that the desired fluorination of S02, HzS, water andthe organic sulfur compounds can be accomplished by passing thecommercial anhydrous hydrogen fluoride in vapor. phase over solid silverdifiuoride or solid cobalt trifluoride. The sulfur dioxide or HEScontent. islthereby reduced to an exceedingly small proportion, thewater is converted to hydrogen fluor'ide and oxygen and the organicsulfur compounds are broken down and converted to inertcompounds, At thesame time the silver difluoride .or cobalt trifluoride is reduced tosilver monofluoride or cobalt difluoride but can beregenerated byfluorination with fluorine gas or chlorine trifluoride. AgFz or 001% maybe used as such in the form of granules or powder but is convenientlymounted on AlFs in the form of porous tablets and these tablets may beused for treatment of anhydrous HF, then subjected to fluorination toconvert the Ag]? or C'oFZ back to AgFz or CoFa and again used fortreatment of HF, the cycle being carried out repeatedly.

Of the heavy metals, the fluorides of which might be expected to behavelike those of silver and cobalt, We have tried out those which arereadily available and cheap enough to be considered non-precious but wefind that because their fluorine is not given up sufficiently readily orpossibly. forre asons we do not understand, they are ineffective toreduce the active sulfur content of AI-IF to the low value which werequire. .OnlyAgFz and COF3 have been found effective to reduce activesulfur to or below .00l% by-weight. V 7

While commercial AHF (anhydrous hydrogen fluoride) normally contains thesulfur impurities noted, the benefit of the invention may be realized toa considerable extent if the AHF fur content of anhydrous HF, which isconvertible to sulfate by HNOs and which is hereinafter referred to asactive sulfur in contradistinction to the inert sulfur compounds SO2F2and SP6, is readily determined by passing the anhydrous HF, containingsmall proportions of such sulfur compounds into nitric acid. Thesecompounds (except S03) are oxidized by the nitric acid and they formH2SO4 therein. The I-I2SO4 is then converted to barium sulfate by theaddition of barium chloride. It is thus possible to measure the amountof active sulfur before and after passing the anhydrous HF over thesilver difluoride or cobalt trifiuoride pellets. been found possible bythe process of the present invention to reduce the active sulfur contentto a low proportion, in the order or" .001% by weight or less as shownby the nitric acid testing method.

Inasmuch as HF has a boiling point of about 19 C. while SO2F2 and 'SFshave boiling points of 52 C. and -62 C. respectively, it is obvious thatby condensation some of these gases may be actually removed from theproduct. It is not, however, essential to remove them in view of theirinert character and, insofar as may be found convenient, they may beleft in the AHF without danger of their corroding shipping drums andequipment. For the production of anhydrous HF of the highest possiblepurity, it may be desirable to follow the treatment of anhydrous HFaccording to the present invention by one or more distillation steps orto pass it repeatedly over the fiuorinating agent or to do both.Absolute purity is, of course, impossible and even where fiuorination ofimpurities is followed by or preceded by one or more distillations, itwill still be desirable to have such slight traces of sulfur as may bepresent, tied up as far as possible in the form of the inert compoundsSO2F2 and SP6.

The temperature of operation according to the invention is not sharplycritical but the reaction proceeds at a more satisfactory rate if thetemperature is elevated. The reaction goes to a substantial extent atroom temperature, but it is desirable that the operation be carried outabove 100 C. and optimum temperature is in the range from 200 C. to 250C. Higher temperatures can be used, but any operating temperature above350 C. is thought unjustifiable in view of the small increase in thereaction rate. The fluorination may be carried out at atmosphericpressure or at higher or lower pressures according to choice.

The silver difluoride or cobalt trifiuoride may be in any desired stateof subdivision but preferably is mounted on a porous aluminum fluoridecarrier in order to give a large area of contact without being carriedalong with the gas stream. Suitable tablets or pellets consisting ofAgF2 or COFs mounted on AlFc may be prepared as follows:

Tablets or pellets of A1203 may be prepared in any suitable manner as,for example, aluminum hydrate after ignition to A1203 powder at 500 C.may be passed through a Stokes pelleting machine. The resulting Altablets may then be impregnated with a water solution of a silver saltor cobalt salt and then fiuorinated by passing, anhydrous HF over suchtablets for a considerable period of time and then passing fluorine. gasover the resulting product whereby to convert the A1203 into AlF3 andthe silver salt or cobalt salt to silver difiuoride or cobalttrifluoride. It is not essential that the entire tablet be so converted,but a small proportion of oxide may be left at the center without dangerthat the tablet It has will crumble in use as would be likely if AgF2 orCoFs were mounted on A1202 tablets. The A1203 tablets can be fiuorinatedwithout the silver or cobalt compound then dipped in a water solution ofa silver or cobalt salt, calcined and then fiuorinated as before. Amixture of A1203 or AlFs and AgCl 0r AgF or C0012 or C05 2 may bepelleted, treated with HF and then treated with fluorine gas. Theproportion of AgF2 in the pellets may be varied but suitably is from 1%to 40 of the weight of the pellet. Numerous variations in themanufacture of the pellets can be tolerated, and accordingly it is notintended to limit the present invention in its broadest aspect to anyparticular fluorinating agent other than as indicated in the appendedclaims.

It is of only limited. economic importance to utilize the pellets to thefullest possible extent prior to regeneration. Rather, it is importantto avoid using the pellets beyond their useful life in the process, andaccordingly regenerations should be more frequent than absolutelynecessary rather than the reverse. The active period betweenregenerations is not sharply limited but toward the end of its usefullife the activity of the pellets falls off fairly rapidly as the silverdifiuoride or cobalt trifluoride becomes converted to silvermonofiuoride or cobalt difluoride. The theoretical weight of silverdifiuoride or cobalt trifluoride required for the conversion of the S02and other active sulfur compounds and water to SO2F2, SP6 and HF can becalculated as well as the amount of fluorine theoretically availablefrom a given weight of pellets, but the pellets should not be expectedto supply more than about 50% by weight of their theoretically availablefluorine before regeneration.

The following specific examples will serve to illustrate the invention;

Erample I A quantity of A1203 was made into tablets of a generallycylindrical shape of a diameter of 0.191. These tablets had an averagecrushing strength of 8 pounds and an apparent density of 1.34. They weredipped in a 20% aqueous solution of AgF for a period of one hour,removed from the solution, placed on the nickel trays of a nickel tubefurnace and treated with a stream of anhydrous HF at 550 C. for 24hours. They were then treated with F2 gas within a temperature rangefrom 150 to 250 C. for 3 hours and were then ready for use. They had anaverage apparent density of 2.14, an average crushing strength of 23.7pounds and assayed 7. 5% as AgF2. Crushing strength is the weightrequired to fracture one tablet between parallel fiat surfaces incontact with opposed portions of the cylindrical surface of the tablet.

Example II A 1.37" inside diameter nickel tube was packed with 340 gramsof the pellets prepared according to Example I. The free space in thiscolumn was calculated to be cc. The temperature in the column wasmaintained at approximately 250 C. HF vapor containing 0.33% S02 waspassed at atmospheric pressure through the tablets at various speeds andfor various times andthe S02, H28 and other active sulfur remaining inthe anhydrous HF after passage through the column was determined by theabove indicated method of absorbing it in nitric acid. Before each run,the pellets were treated at 250 C. with fluorine gas for regeneration tofull strength.

An excess of F2 was employed in order to insure complete conversion ofAg]? to AgF2. The resultsof these runs are tabulated below: p

SO-z, Per SO J Per HF Flow, Time or ai gg centby cent by gms./min. Flow,Min. welght weight Feed product 1 .45 130 58.5 0.33 0.002 2 1.14 .00 s.4 0. 33 0.0008 a- 1. 02 40 64.8 0. 33 0000s 4 2. 5s 30 02. 9 0. s3 0.0000 4. 0s 24 112. 3 0. 33 0. 0000 11. 1 22 244. 2 0. 33 2 0. 002 14. 410 230. 4 0. 3a 0. 0000 1 This value, assumed to be entirely S02 butcontaining traces of HES and organic sulfur, was determined by passing almown Weight of HF into HNO3, measuring the H2804 formed in the nitricacid and calculating to S02.

2 The variations in runs 1 and 6 from the other rims are not consideredsignificant but are within the probable error of the experiment. Thepercentage of S00 plus other active sulfur remaining in the anhydrous HFmay be considered to be in the order of 0.001% for all runs.

Example III temperature of 250 C. The resulting tablets weighed 3560 g.and assayed 19.5% AgF2.

Example IV The reactor described in Example II was loaded with 445 g. ofthe pellets prepared according to Example III, and a series ofexperiments similar to those described in Example II were carried out.The free space in this column was calculated to be 143 cc. The resultsof these runs are tabulated below:

S02, Per 802, Per

R HF Flow, Time of 235 5 55 cent by cent by un gms./mi.n. Flow, Min. gIted) Weight weight Feed product 1 This value, assumed to be entirelyS02 but containing traces of HzS and organic sulfur, was determined bypassing a known Weight of HF into HNO3, measuring the H280 formed in thenitric acid and calculating to S02.

Example V A 1.87 inside diameter nickel tube was packed with 1180 g. ofCoF3A1F3 tablets which had been prepared according to'the methoddescribed in Example III except that a 50% Co(NOa) 2 solution was usedin place of the AgNOa solution. The tablets assayed 10.8% COFs and thefree space in the column was calculated to be 420 ml. AI-IF was passedthrough the column at various rates and temperatures. The results ofthese runs are tabulated below:

S02 Per S02 Per HF Flow Total HF cent by Run Temp. O. cent by g. min.gins. Welght I Feed Product Having thus described our invention, what weclaimis:

1. A process forconditioning anhydrous hy-' drogen fluoride, containingsulfur dioxide as an impurity, the step of passing such anhydroushydrogen fluoride over a mass of solid, heavy metal fluoridefluorinating agent of the class consisting of AgFz and COF3, saidfluorinating agent being extended on the surface of a porous aluminumfluoride carrier, the flow of anhydrous hydrogen fluorideover saidfluorinating agent being continued until a substantial proportion butless than all of the said fluorinating agent has been reduced to acompound having one less fluorine atom, and stopping the flow ofanhydrous hydrogen fluoride before the total amount thereof passed oversaid mass of fluorinating agent is such that its total sulfur content isequal to the amount theoretically required to reduce all of saidfluorinating agent to a compound having one less atom of fluorine.

2. A process for conditioning anhydrous hydrogen fluoride, containingsulfur dioxide as an impurity, the step of passing such anhydroushydrogen fluoride in vapor phase over a mass of solid, heavy metalfluoride fluorinating agent of the class consisting of AgFz and COFs,said fluorinating agent being extended on the surface of a porousaluminum fluoride carrier, the flow of anhydrous hydrogen fluoride oversaid fluorinating agent being continued until a substantial proportionbut less than 50 of the said fluorinating agent has been reduced to acompound having one less fluorine atom, and stopping the flow ofanhydrous hydrogen fluoride before the total amount thereof passed oversaid mass of fluorinating agent is such that its total sulfur content isequal to 50% of the amount theoretically required to reduce all of saidfluorinating agent to a compound having one less atom of fluorine.

3. In a process for conditioning substantially anhydrous hydrogenfluoride containing sulfur dioxide as an impurity, the step of passingsuch anhydrous hydrogen fluoride in vapor phase over a mass of solidparticles of a fluorinating agent composed of aluminum fluoride as acarrier and silver difluoride as a fluorinating agent extended on thesurface of said carrier, the flow of anhydrous hydrogen fluoride oversaid fluorinating agent being continued until a substantial amountthereof has been conditioned to reduce the sulfur content below 0.002%by weight, and discontinuing the flow before the total amount of sulfurremoved fromsaid anhydrous hydrogen fluoride is equal to 50% of thetheoretical amount required to reduce all of said fluorinating agent toAgF, the total amount of anhydrous HF passed over said fluorinatingagent being less than an amount having a sulfur content equal to 50% ofthe amount theoretically required to reduce all the AgF2 to AgF.

4. In a process for conditioning substantially anhydrous hydrogenfluoride containing sulfur dioxide as an impurity, the step of passingsuch anhydrous hydrogen fluoride in vapor phase over a mass of solidparticles of a fluorinating agent composed of aluminum fluoride as acarrier and cobalt trifluoride as a fluorinating agent extended on thesurface of said carrier, the floW of anhydrous hydrogen fluoride oversaid fluorinating agent being continued until a substantial amountthereof has been conditioned to reduce the sulfur content below 0.002%by weight, and discontinuing the flow before the total amount of sulfurremoved from said anhydrous hydrogen fluoride is equal to 50% of thetheoretical amount re- 8 quired to reduce all of said fiuorinating agentto UNITED STATES PATENTS COFz, the total amount of anhydrous HF passedNumber Name Date over said fluorinating agent being less than an 2 382788 .Gorin Aug 14 5;. amount having a sulfur content equal to 50% of uthe amount theoretically required to reduce all 5 OTHER REFERENCES theCOFs to CoFz. Ruff et al.: Z. Anorg. und Allg. Chem; 219,

WILBUR J. SHENK, 147-148 (1934). GEORGE R. PELLON- Gmelin-Kraut:Handbuch der Anorganisch m Chemie, 1. 2, Heidelberg, 1909, p. 15.REFERENCES CITED 10 Benner et al.: Ind. and Eng. Chemistry, vol.

The following references are of record in the 9, March 1 D- file of thispatent:

1. A PROCESS FOR CONDITIONING ANHYDROUS HYDROGEN FLUORIDE, CONTAININGSULFUR DIOXIDE AS AN IMPURITY, THE STEP OF PASSING SUCH ANHYDROUSHYDROGEN FLUORIDE OVER A MASS OF SOLID, HEAVY METAL FLUORIDEFLUORINATING AGENT OF THE CLASS CONSISTING OF AGF2 AND COF3, SAIDFLUORINATING AGENT BEING EXTENDED ON THE SURFACE OF A POROUS ALUMINUMFLUORIDE CARRIER, THE FLOW OF ANHYDROUS HYDROGEN FLUORIDE OVER SAIDFLUORINATING AGENT BEING CONTINUED UNTIL A SUBSTANTIAL PROPORTION BUTLESS THAN ALL OF THE SAID FLUORINATING AGENT HAS BEEN REDUCED TO ACOMPOUND HAVING ONE LESS FLUORINE ATOM, AND STOPPING THE FLOW OFANHYDROUS HYDROGEN FLUORIDE BEFORE THE TOTAL AMOUNT THEREOF PASSED OVERSAID MASS OF FLUORINATING AGENT IS SUCH THAT ITS TOTAL SULFUR CONTENT ISEQUAL TO THE AMOUNT THEORETICALLY REQUIRED TO REDUCE ALL OF SAIDFLUORINATING AGENT TO A COMPOUND HAVING ONE LESS ATOM OF FLUORINE.